(1) Field of the Invention
The present invention relates to a method of fabricating trench isolation, and more particularly to a process for fabricating trench isolation to solve the issue of kink effect.
(2) Description of the Related Art
In recent years, the sizes of the MOSFETs and capacitors have become continuously smaller so that the packing densities of these ICs have increased considerable. It is necessary to isolate all of the transistors in order to prevent the transistors affecting one another. At the early stage, the method of Local Oxidation (LOCOS) was used as the isolation process. The method of LOCOS uses thermal oxidation to form a field oxide layer to isolate all of the transistor. To use LOCOS as isolation process has a serious drawback--bird's beak. The bird's beak, the lateral extension of field oxide during thermal oxidation process, will affect the subsequent process at the active regions. This issue is getting serious in the field of sub-micron and deep sub-micron technology. Therefore, a lot of new isolation processes were developed to substitute the method of LOCOS.
Trench isolation is the first candidate to substitute LOCOS. FIG. 1 is a layout diagram with trench isolation, including gate (G), source (S), and drain (D). FIG. 2 schematically illustrates the cross section of FIG. 1 in the direction of AA. Referring now to FIG. 2A, areas of trench isolation 200 are formed on a silicon substrate 1 by the method of photolithography and etching, and then the areas of trench isolation 200 are filled by depositing an oxide layer 2. After that, a process of planarization is performed to remove the extra oxide.
Referring now to FIG. 2B, the surface of the oxide at the areas of trench isolation is lower than the surface of the oxide on the substrate. After the process of planarization to remove the extra oxide, the oxide at the edges of the active regions 500 is totally removed. As a result, the edges of the active regions 500 are no more covered by oxide. Because the edges of the active regions are in the shape of right angle, their radius of curvature is very small. Therefore, the surface energy at the edges is very high, and it's difficult to form a thin film at the edges. During the subsequent process of thermal oxidation to form the gate oxide at the active regions, the thickness of the gate oxide 100 at the corners will be much less than that at the active regions. Consequently, the kink effect will happen at the resulting transistors. The so-called kink effect is a phenomenon of a transistor turning on abnormally, and thus affects the performance and the yield of ICs.
In some prior arts, an oxide layer and a nitride layer are formed as a hard mask to protect the active regions before defining the areas of trench isolation [don't show in FIG. 2 (please refer to J. M. Boyd & J. P. Ellul, J. Electrochem. Soc., 143(11), 3718(1996) for more details)]. The nitride layer is performed as a stop layer, in order to prevent removing too much oxide 2a at the areas of trench isolation during the process of etching oxide 2 back. However, the oxide 2a will also be attacked during the subsequent process of removing the nitride layer. As a result, the edges of the active regions are no more covered by oxide, and are thus in the shape of right angle.
Therefore, the present invention discloses an easy and manufacturable method to fabricate trench isolation for high density IC applications.